Could you please give an explanation on the concept of furnace (炉) and crucible (鼎) and their function?

Such terms, as you can see, often appear here and there in Taoist alchemy books, including the book Understanding the Reality (悟真篇) with the following excerpt (translation by Thomas Cleary):

Setting up furnace and crucible is patterned on the Heaven and Earth;
Refining the sunlight and moonlight stablizes the Yang and Yin souls.
Congealing and dissolving, the incubating warmth produces transmutation;